Various types of single-ended fluorescent lamps have been proposed, see, for example, the referenced U.S. patent application Ser. No. 415,451, filed Sept. 7, 1982, STEEGER et al., and descriptive literature such as "Popular Science", November 1983, pages 22, 24, 26 "FOLDED FLUORESCENTS", article by David Scott and Susan Renner-Smith. Single-ended fluorescent lamps, referred to as compact fluorescent lamps, have substantial advantages with respect to the customary elongated fluorescent lamps due to their small space requirements. A ballast and starting circuit can be constructed small enough so that it is retained within the base construction, or a small base extension. These lamps, then, can be fitted with customary "Edison" screw threads and used as direct replacement for incandescent lamps of various wattage ratings. Compact lamps of this type, due to their special construction, are thus suitable for direct replacement of customarily used incandescent lamps but, due to their substiantially higher conversion efficiency from electrical power into light energy, and their longer life, the lamps are substantially more economical in use. Compact lamps of this type did not, however, as yet achieve the market acceptance which their advantages would indicate due to the high initial cost thereof. The internal construction of these lamps still is too complex, resulting in excessive manufacturing costs, and, consequently, eventual cost to the user for the lamp itself which is above the threshold of commercial acceptance. Lamps of simpler construction do not have the substantially higher electrical-to light conversion efficiency, so that, even though the price may be less, the incentive for their use is also less. Development of compact fluorescent lamps thus has an aim to provide for high electrical-to-light conversion efficiency, a light color spectrum which is acceptable for universal use and renders color illuminated by the lamp essentially with the same quality as daylight, or acceptable fluorescent lamps, and additionally permits construction with respect to size and shape approaching that of incandescent lamps, while permitting manufacture so that the lamps can be sold at a price level which is competitive with incandescent lamps.
It has previously been proposed--see German Patent Disclosure Document No. DE-OS 28 35 574, to which U.S. Pat. No. 4,199,708 corresponds--to construct a single-ended low-pressure lamp in which an outer bulb surrounds a preheatable electrode which is located within one end of a generally U-shaped tube located within the outer bulb. The electrode is included in the U-shaped tube by a gas-tight melt. Gas-tight connection between the U-shaped inner tube and the preheatable electrode, as well as with the outer bulb which includes a gas filling, is made by using a connecting melt glass which is bonded to a chromium steel plate, forming a base section. The various components of the lamp, soldered or otherwise bonded together, require a plurality of manufacturing steps which are expensive, resulting in overall high cost of the lamps. In mass production, it has been found that the multiplicity of melt connections may introduce manufacturing defects, so that the overall reject rate of the lamps, due to possibly slightly leaking joints, was excessive and beyond acceptable commercial levels.
It has also been proposed to construct single-ended fluorescent lamps by using a pre-formed accepting plate which has an axial extension in form of a stem, extending within the bulb itself. The electrode and the U-shaped inner tube are secured to this element, and the outer bulb is then melt-connected to the dish-shaped side of the base-and-stem unit formed thereby.
This construction, like the one utilizing a metal base, has an additional disadvantage: The electrodes will be positioned above the edge of the base. The space surrounding the electrodes, as well known in connection with fluorescent lamps, may become blackened. This blackened area then will be visible. The open end of the U-shaped inner tube will be above the edge of the base. If no fluorescent coating is applied, for example, to the inner surface of the outer bulb, the blueish mercury vapor discharge will be visible when the lamp is in operation, which is undesirable due to distortion of color rendition of light emitted from the lamp and interferes with consumer acceptance thereof.
Various lamps of this type use not only one, but a pluarlity of inner tubes, for example two, which are located parallel to each other within the interior of the bulb.